In methods of chronic blood-cleaning therapy, for example in hemodialysis, hemofiltration, and hemodiafiltration blood is conveyed via an extracorporeal blood circuit. An arteriovenous fistula is often applied surgically as an access to the blood vessel system. The use of an implant is also possible.
Of importance to the functional capability of fistulas is their perfusion. If the fistula flow falls below a critical value, the risk of a fistula thrombosis then increases with the possible loss of the vascular access, which in dialysis treatment represents a considerable complication. If the fistula flow during the dialysis treatment is insufficient and smaller than the extracorporeal blood flow, local fistula recirculation occurs, whereby a fraction of the dialysed blood fed back to the fistula with the venous blood line is again fed to the dialyser via the arterial blood line. The fistula recirculation causes a significant reduction in the dialysis efficiency (F. Gotch, “Models to predict recirculation and its effects on treatment time in single-needle-dialysis”, First Intl. Symposium on Single-Needle-Dialysis, S. Rignoir, R. Vanholder and P. Ivanovich, Cleveland, ISAO Press, 1984, page 305 ff.). The measurement of the quality of the vascular access is therefore regarded as an important means of quality assurance in dialysis treatment.
Apart from fistula recirculation, a small part of the blood with such vascular accesses always circulates directly via the patient's blood circulation to the vascular access without being able to participate in the metabolic process in the capillary systems. This type of recirculation is called cardiopulmonary recirculation and usually amounts to a few percent.
On account of their clinical importance, a number of methods are known for measuring the recirculation. The known methods are generally based on a measurement of a physical or chemical characteristic quantity of the blood, which is changed in the venous branch of the extracorporeal circuit. The physical or chemical characteristic quantity of the blood may be changed directly by manual intervention in the extracorporeal blood circuit or also indirectly by intervention in the dialysing fluid circuit.
A method for the measurement of recirculation referred to as thermodilution is known from the article by Krämer and Polaschegg in the EDTNA-ERCA Journal Vol. XIX, no. 2, pages 8-15, April 1993. With the known method, a brief drop in temperature is initiated in the dialysing fluid circuit, which is transferred to the venous branch of the extracorporeal blood circuit and leads to a detectable temperature jump in the arterial branch of the extracorporeal circuit when a recirculation occurs.
A known device for performing the method referred to as thermodilution has a temperature sensor arranged in the arterial branch and venous branch of the extracorporeal circuit. The venous temperature sensor is used to detect the temperature jump that is attributable to the drop in temperature produced in the dialysing fluid circuit. The measured temperature jump is integrated over time and subsequently compared with the temperature course recorded in the arterial measuring sensor. The ratio of the two temperature integrals with respect to one another is a measure of the overall reduction in efficiency of the dialysis treatment due to fistula and cardiopulmonary recirculation.
The known device for the measurement of recirculation has been tried and tested in practice. A decisive drawback, however, lies in the fact that the recirculation is determined on the basis of a measured change in a physical or chemical characteristic quantity of the blood in the blood circuit. This is disadvantageous inasmuch as temperature sensors have to be arranged in the blood circuit. A relatively high outlay on equipment is thus incurred. Furthermore, the measurement with the temperature bolus requires a relatively long time. The recirculation measurement with the temperature bolus is therefore not carried out continuously during the treatment, but often only upon manual request.
German patent document DE 197 02 441 shows the drawbacks of the method referred to as thermodilation. In order to reduce the equipment outlay for the extracorporeal blood treatment apparatus, German patent document DE 197 02 441 proposes a method for determining the recirculation on the basis of the change in a physical or chemical characteristic quantity in the dialysing fluid upstream of the dialyser and detecting the change in a physical or chemical characteristic quantity in the dialysing fluid downstream of the dialyser.
Apart from the recirculation measurement, various methods are known for measuring the dialysance or clearance, which are criteria for the efficiency of a dialysis treatment. U.S. Pat. No. 6,702,774 describes a method for measuring dialysance or clearance during an extracorporeal blood treatment. In contrast with the method for measuring recirculation known as thermodilution, the determination of the dialysance or clearance takes place on the basis of a measured physical or chemical characteristic quantity of the dialysing fluid in the dialysing fluid circuit. A measurement in the extracorporeal blood circuit is not therefore required.
With the known methods, a physical or chemical characteristic quantity of the dialysing fluid is changed in a preset time interval upstream of the dialyser and the change in the physical or chemical characteristic quantity is measured downstream of the dialyser. The physical or chemical characteristic quantity is preferably the ion concentration of the dialysing fluid, which is determined by a conductivity measurement of the dialysing fluid. The dialysance is determined solely from the quantities on the dialysing fluid side from the integral over time of the measured dialysing fluid ion concentration upstream of the dialyser and the integral over time of the measured ion concentration downstream of the dialyser.
German patent document DE 197 47 360 A1 describes a method for determining the dialysance and clearance, wherein a predetermined quantity of a substance is added as a bolus to the dialysing fluid upstream of the dialyser, the dialysance of which substance is to be determined. The quantity of substance not dialysed in the dialyser is determined over time by integration of the substance concentration measured with a sensor downstream of the dialyser and the dialysance is ascertained from the added quantity of substance, the quantity of substance detected downstream of the dialyser and the dialysing fluid flow. German patent document DE 197 47 360 A1 mentions as the prior art the method described in German patent document DE 39 38 662, with which only the effective clearance, but not the dialyser clearance may be measured, whereby a distinction between the two values is to be made by the influence of the recirculation.